CCD transversal filters of the output-weighted type use a tapped CCD delay line to provide differentially delayed responses to a succession of input signal samples, which are then linearly combined. Linear combining comprises the weighting of the responses and the additive or subtractive combining of them. The tapped CCD delay line used in a CCD transversal filter has to use non-destructive charge sensing stages at the successive taps along its length, so that the sensed charge packets can continue to be clocked forward along the line after sensing.
The prior art method of tapping the delay line involves the use of "floating" gate electrodes in the succession of gate electrodes along the length of the CCD delay line. These floating gates connect to the gate electrodes of field effect transistors (FET's) in common-drain or common-source amplifier connection. The FET's serve as electrometers sensing the amplitudes of charge packets to provide output signal current or voltage samples. These floating gates must be periodically clamped to known potential to restore dc to the electrometer output signals. This clamping is customarily done with a respective field effect transistor for each floating gate, each of which transistors is in transmission gate connection from the floating gate electrode associated therewith to a source of reset potential. The amount of area under the gate structure comprising the floating gate, the gate electrode of the electrometer FET, and the end contact to the conduction channel of the dc-restoration clamp FET tends to be substantial. So the capacitance C of the gate structure is appreciable, and there is consequently substantial Johnson noise (dependent on C.sup.(1/2)) associated with the electrometer output from each CCD delay line tap.
The lowest-noise charge sensing stage available for CCD's is the floating diffusion electrometer. In this electrometer the gate electrode of the electrometer FET connects to a floating diffusion in the CCD charge transfer channel, and the clamping of the gate electrode to reference potential for dc restoration is done by FET action between the floating diffusion itself and a reset drain diffusion connected to the reference potential. The conduction channel for reset clamping is induced in the charge transfer channel by a reset pulse applied to a reset gate electrode positioned between the floating diffusion and a reset drain diffusion defining the end of the conduction channel. The gate electrode structure connected to the floating diffusion is smaller because it does not have to be ohmically contacted to a channel electrode of a clamp FET located outside the charge transfer channel. The capacitance C and thus the Johnson noise of the gate structure are appreciably reduced from what they are with the floating-gate electrometer.
L. N. Davy in his U.S. Pat. No. 4,330,753, issued May 18, 1982 and entitled "METHOD AND APPARATUS FOR A SIGNAL FROM A CHARGE TRANSFER DEVICE" describes a method for obtaining what he characterizes as relatively noise-free information signals from the output stage of a charge transfer device. In the method Davy describes, the output signal from the regularly sampling electrometer stage is passed through a band-pass filter to separate double-sideband amplitude-modulation sidebands flanking a harmonic of the clocking frequency of the electrometer stage. These side-bands are then synchronously detected, using a switching demodulator operated at the harmonic of the clocking frequency to heterodyne the sidebands down to the baseband spectrum. This synchronous detection discards response to flicker noise, or 1/f noise, in the baseband spectrum of the electrometer stage response.
The operation of a floating-diffusion electrometer at video-rate clocking frequency has been attempted without resetting the floating diffusion, but this operation was not followed with a subsequent synchronous detection process. This operation without resetting the floating diffusion or synchronously detecting the electrometer response results in a smearing of charge packets into succeeding charge packets. The smearing arises because of the inefficiency in charge transfer attending the omission of resetting the floating diffusion. The present inventor believes himself the first to realize that this smearing, while it ruins baseband spectral response in the lower frequencies particularly, does not appreciably affect spectral response in the sidebands of the electrometer stage clocking frequency. In U.S. patent application Ser. No. 525,491, filed Aug. 22, 1983, and now abandoned entitled "REDUCTION OF NOISE IN RECOVERY OF SIGNAL FROM CHARGE TRANSFER DEVICES", and assigned to RCA Corporation, the present inventor describes the floating diffusion as being reset to an in-channel potential, rather than to reset drain potential. The smear in the electrometer output is then removed by low-frequency suppression filtering, and the filter response is synchronously detected at electrometer stage clocking frequency to obtain a smear-free output signal.